Recently, lithium secondary batteries have been used in various fields including portable electronic devices, such as mobile phones, personal digital assistants (PDAs), and laptop computers. In particular, in line with growing concerns about environmental issues, research into lithium secondary batteries having high energy density and discharge voltage as a power source of an electric vehicle, which may replace vehicles using fossil fuels such as gasoline vehicle and diesel vehicle, one of major causes of air pollution, has been actively conducted and some of the research are in a commercialization stage. In order to use a lithium secondary battery as a power source of the electric vehicle, the lithium secondary battery must maintain stable power in a usable state of charge (SOC) range along with high power.
With respect to a typical cathode material of LiCoO2, as a cathode material of a lithium secondary battery for realizing high capacity, practical limits of an increase in energy density and power characteristics have been reached. In particular, when LiCoO2 is used in high energy density applications, oxygen in a structure of LiCoO2 is discharged along with structural degeneration in a high-temperature charged state due to its structural instability to generate an exothermic reaction with an electrolyte in a battery, and thus, it becomes a main cause of battery explosion. In order to improve the safety limitation of LiCoO2, the use of lithium-containing manganese oxides, such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxide (LiNiO2) have been considered, and a great deal of research into using ternary layered oxide of LiNixMnyCo1-x-zO2 (hereinafter, referred to as “NMC”) has recently been conducted.
Nickel (Ni) in Li[Ni1/3Co1/3Mn1/3]O2, which is the most representative among the NMC, may change from Ni+2 to Ni+3 or Ni+4 according to SOC during charge. In this case, Ni+3 or Ni+4 (particularly, Ni+4), different from stable Ni+2, may lose lattice oxygen due to its instability to be reduced into Ni+2, and the lattice oxygen may change surface properties of an electrode or may increase a charge transfer impedance of the surface by reacting with an electrolyte solution to reduce capacity or degrade high-rate capability.
In order to address such limitations of NMC, research into mixing olivine-structured lithium oxide, for example, LiFePO4 (hereinafter, referred to as “LFP”) with the NMC has been conducted.
However, in a case where LFP is mixed with NMC, a rapid voltage drop may occur near 3.4 V to 3.6 V during discharge due to the difference in operating voltage.
In order to address the above limitations, a method of mixing the LFP with lithium manganese oxide (hereinafter, referred to as “Mn-rich”), in which manganese (Mn) as an essential transition metal is added to layered-structure lithium manganese oxide as a high-capacity material in a larger amount than other transition metals (excluding lithium), may be considered.
However, in a case where Mn-rich and LFP are blended, since the operating voltage of the Mn-rich is generally lower than NMC, LFP may early participate in discharge after discharge of about 50%. Thus, the Mn-rich may be difficult to assist power at a lower end of SOC. A portion mainly requiring the power assistance as resistance increases in the lower end of SOC may be a SOC range of about 10% to 40%. However, a voltage in this range becomes lower than that of LFP. That is, in the case of blending of NMC and LFP, since the LFP may be discharged after the discharge of the NMC is completed (apart from the above-described rapid voltage drop near 3.4 V to 3.6 V), the LFP may compensate for power reduction due to the increase in resistance at the lower end of SOC (SOC range of 10% to 40%). However, in the case of blending of Mn-rich and LFP, the LFP may not compensate for the power reduction.
Thus, with respect to a cathode material including Mn-rich, there is an urgent need to develop a new cathode material capable of widening an available SOC range by alleviating the rapid power reduction in a low SOC range (e.g., a SOC range of 10% to 40%) as well as providing improved safety.